NASA's MESSENGER spacecraft orbited Mercury from 2011 to 2015 and has provided new insights into the interior of the innermost planet. Mercury has a large metallic core ~2,000 km in radius covered by a thin layer of rock only ~420 km thick. Furthermore, a surprisingly large fraction of this outer layer was produced by melting of deeper rocks, forming a light crust ~35 km thick. The core is now known to produce a magnetic field that has intriguing similarities and differences compared to Earth's field. Some rocks near the surface are magnetized, and the strongest magnetizations are likely to be >3.5 billion years old. This new understanding of Mercury's interior is helping reveal how rocky planets operate.

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